1. Field of the Invention
The present invention relates to a process of reducing nitrogen oxide emissions from combustion devices.
2. Description of the Prior Art
In the combustion of fuels with fixed nitrogen such as coal, oxygen from the air may combine with the fixed nitrogen to produce nitrogen oxide NO or nitrogen dioxide NO.sub.2 which are collectively called nitrogen oxides or NO.sub.x. At sufficiently high temperatures, oxygen reacts with atmospheric nitrogen to form nitrogen oxides. So, even fuels that have no fixed nitrogen produce nitrogen oxides when fired with excess air at sufficiently high temperatures. Production of nitrogen oxides is regarded as undesirable. There are numerous government regulations which limit the amount of nitrogen oxides which may be emitted from combustion devices. Furthermore, the presence of nitrogen oxides in a flue gas causes the condensate formed from the gases to become more corrosive and acidic. Consequently, there is a need for apparatus and processes which reduce the nitrogen oxide emissions in flue gas.
Numerous attempts have been made to develop apparatus and processes which reduce the nitrogen oxide emissions from combustion devices. Among the attempts are burner redesigns such as dual register low NO.sub.x burners, staged air combustion, flue gas recirculation, reduced air preheat, and increased furnace size. All of these techniques have had some success, but all suffer from some limitations and they are expensive. Currently more stringent regulations are forcing the development of new processes for NO.sub.x control.
One such new approach is a process known as in furnace NO.sub.x reduction, reburning, or fuel staging. In reburning, coal, oil, or gas is injected above the normal flame zone to form a fuel-rich zone. In this zone, part of the nitrogen oxides are reduced to ammonia-like and cyanide like fragments which are then oxidized to form N.sub.2 and nitrogen oxide.
Several problems occur when this process is used. First, coal may be an inefficient reburn fuel because of its high fixed nitrogen composition. The fixed nitrogen introduced at this location in the furnace will have less chance of being converted to N.sub.2, and therefor have a higher chance of ending up as nitrogen oxides and may, depending on the nitrogen oxide concentration of the flue gas, increase the emissions of nitrogen oxides.
Furthermore, the fuel must be injected with a sufficient volume of gas. If air is used as this gas, there must be enough fuel to consume the oxygen in the flue gas and air, and to supply an excess of fuel so reducing conditions exist. This increases the amount of fuel which must be used as reburn fuel. Furthermore, the necessity of using carrier air requires extensive duct work in the upper part of the furnace.
Additionally, the reburn fuel must be injected well above the primary combustion zone of the furnace so that it will not interfere with the reactions taking place therein. However, this fuel must be made to burn out completely without leaving a large amount of unburned carbon. To do this, the fuel must be injected in a very hot region of the furnace some distance from the furnace exit. The exit temperature of the furnace must be limited in order to preserve the heat exchanger's surface. Therefore, a tall furnace is required to complete this second stage process.
Moreover, the fuel must be injected in quantities sufficient to make the upper furnace zone fuel-rich. This excess fuel ultimately requires more air in order to be completely combusted. Thus, air must be injected above the zone of reburn fuel injection. This arrangement requires even more duct work and furnace volume. This excess air must consume the reburn fuel before the gases reach the furnace entrance.
Finally, most coal furnaces which are now in operation are not designed to accommodate the prior art methods. Major modifications such as the provision of extensive duct work and the addition of a second stage to the process are required to utilize the prior art method. Such retrofitting is expensive. Consequently, there is a need for a combustion process requiring little additional apparatus which will reduce nitrogen oxide emissions in flue gas and which can be readily used in existing furnaces.